Recently, a technique for manufacturing a semiconductor device, for example a thin film transistor (hereinafter, a TFT) over a substrate has made progress considerably, and application development to an active matrix type display device, for example, has made. Specifically, a TFT using a crystalline semiconductor film can be operated at high speed since the TFT has a higher electric field effect mobility (also referred to as mobility) as compared with a TFT using a conventional amorphous semiconductor film. Therefore, control of a pixel which has done by a drive circuit provided outside the substrate has been tried to be performed by a drive circuit formed over the same substrate as the pixel.
A glass substrate is more promising entry than a quartz substrate as a substrate in a semiconductor device from a monetary viewpoint. The glass substrate is inferior in a heat resistance property and heat deformation is easily generated. Therefore in the case of forming a TFT using a crystalline semiconductor film over a glass substrate, laser annealing is used for crystallizing a semiconductor film so as to avoid the heat deformation of the glass substrate.
The feature of the laser annealing is that a processing time can be vastly shortened compared to an annealing method in which radiant heating or conductive heating is used, and the substrate is not damaged practically by heating a semiconductor substrate or a semiconductor film selectively and locally.
Here, the laser annealing method refers to a technique for crystallizing a damaged layer or an amorphous layer formed over a semiconductor substrate or a semiconductor film, a technique for crystallizing an amorphous semiconductor film formed over a substrate, and a technique for heating (annealing) a crystalline semiconductor film which is not a single crystal (a semiconductor film which is not the above mentioned single crystal is collectively referred to as an amorphous semiconductor film). Further, a technique to be applied for planarization and surface modification of a semiconductor substrate or a semiconductor film is included.
Excimer laser is often used for laser annealing. The excimer laser has advantages that it has high output and it can perform repeated irradiation with high frequency. Moreover, the laser beam emitted from the excimer laser has an advantage that it is sufficiently absorbed in a silicon film which is often employed as the semiconductor film.
As for the laser beam irradiation, the shape of the laser beam at an irradiation surface is formed to be linear by an optical system; an irradiation position of the laser beam is moved relatively to the irradiation surface in a width direction of the linear laser beam (referred to as a linear beam). Thus, a method for the irradiation has high productivity and is superior to industrial use (refer to patent document 1).    [Patent Documents 1]    Japanese Patent Laid-Open No. 8-195357